Time and tide: Mapping the changing 3D shape of Australia's dynamic intertidal zone using time series satellite data

The intertidal zone is the dynamic boundary between land and sea, where regular tidal inundation creates complex coastal environments that support critical ecosystem services and ecological habitats. Despite their importance and vulnerability to climate change and coastal development, data describing the spatial extent and three-dimensional structure of intertidal environments remain limited. Recent advances in Earth Observation (EO) have enabled the first continental to global-scale maps of intertidal extent and elevation. However, existing approaches are constrained by coarse spatial detail, limited scalability, or static representations of highly dynamic coastal systems. Here, we present Digital Earth Australia (DEA) Intertidal, a new EO method for mapping three-dimensional intertidal elevation through time and at continental scale. The approach integrates multi-sensor Landsat and Sentinel-2 data with global tide modelling to analyse subtle, tide-driven changes in terrain wetness at the pixel level. This pixel-based approach requires no in-situ calibration data, and enables extraction of fine-scale morphological detail that cannot be resolved using traditional waterline-based approaches. We demonstrate our approach with a continental-scale application across the entire Australian coastline, encompassing some of the world's most complex tidal regimes and extensive intertidal habitats. By validating our results against a large, multi-temporal independent dataset, we demonstrate consistently high performance across diverse tidal environments and highlight key strengths and limitations of the approach. Compared to previous continental-scale efforts, DEA Intertidal delivers improved spatial detail while requiring substantially fewer temporal observations to generate accurate elevation models. The resulting time series elevation and uncertainty datasets reveal geomorphologically significant coastal features, and enable quantitative analysis of vertical, horizontal, and volumetric coastal change at scale. Built entirely on open satellite data, global tide models, and open-source processing frameworks, DEA Intertidal provides a scalable and transferable approach for monitoring intertidal dynamics globally.